This invention is concerned with a system for delivering anesthetics and analgesics. More particularly, the invention is concerned with a method for delivering anesthetic and analgesic inhalation gases. When used in a broad sense hereinafter and in the claims, "anesthetic" is intended to include the administration of inhalation gases which are both anethetic and/or analgesic.
Present delivery systems for inhalation anesthetics utilize the principle that a volatile liquid maintains a constant vapor pressure in the gas phase above the liquid. Anesthesia machines are constructed so that a stream of carrier gas passes through a baffled mixing chamber containing a few hundred milliliters of the liquid anesthetic. This stream becomes saturated with anesthetic vapors and the concentration is determined exclusively by the temperature (the higher the temperature, the higher the anesthetic concentration in the saturated vapors). The saturated stream is then mixed with a controlled amount of oxygen and/or nitrous oxide to dilute the saturated stream to a known concentration which is then delivered to the patient. The anesthesiologist determines the concentration from the temperature and by adjustment of a complex series of valves and flow meters.
Since each anesthetic has a different vapor pressure, a recalibration or even replacement of the vaporizer is necessary each time the anesthetic is changed.
Anesthetic concentrations of 0.5-4% are generally employed. Recommended flow rates for proper functioning of the equipment are in the 2-4liters/min range. However, flow rates of 1000 cc/min are more desirable since this is the rate of oxygen uptake by the patient. The reduction of flow rates from 2-4 liters/min to 200-400 cc/min is commonly accomplished by a pop-off valve usually located on a surge or breathing bag.
The present methods of delivering inhalation anesthetics possess a number of substantial drawbacks (some of them inherent) which although recognized by the large majority of anesthesiologists have heretofore not been resolved in a satisfactory manner. These disadvantages include the inability of present metering devices, particularly in recycling systems, to effectively provide desired concentrations of anesthetics at less than 2 liters/min. flow rates. As mentioned hereinbefore the decrease in flow rates is typically effected by a pop-off valve. Unfortunately pop-off valves commonly release the anesthesia into the operating room itself, which is further disadvantageous since it may cause the operating surgeon to become drowsy at times when full concentration is absolutely essential. It is also suspected that prolonged exposure of operating personnel to anesthetics such as halogenated ethane may give rise to kidney and liver problems. Additionally, there has been alleged a relationship between exposure to inhalation of anesthetics and the high statistical rate of miscarriages experienced by operating room nurses. Venting inside and outside the operating room has been known to contaminate the closed circulatory air system of hospitals causing air pollution within the hospital. Regardless however of where the anesthesia is vented, excessive flow rates which require venting cause a considerable waste since up to 90% of the anesthetic gas delivered is unused.
A further difficulty arises when the exhaled gas is fed back via a closed recycling system eventually into the surge or breathing bag. The in-put of gas containing an unknown concentration of unused anesthetic obviously impairs the anesthesiologists's ability to establish the influent concentration of anesthesia that the patient inhales. Although the experienced anesthesiologist carefully detemines the unconscious state of the patient during an operation by monitoring the patient's vital signs such as heart beat, etc., it would obviously be of great assistance for him to know exactly what amount of anesthetic the patient inhales at any given time and just as importantly for him to possess the ability to control precisely the amount of anesthetic supplied to the patient.
A further drawback of today's complicated anesthesia machines is their high maintenance cost. They are usually scheduled weekly for cleaning and "degumming" of the mixing chamber and ancillary equipment. Conscientious anesthesiologists frequently feel obligated to personally perform this function. A system which obviates the cleaning would therefore result in considerable savings of valuable time.
Inhalation anesthetics are sometimes used outside of hospital operating rooms, such as for example in dentists' offices. Dentists frequently use local analgesics by injection although they would prefer inhalation analgesics were it not for the cost of today's complex anesthesia machines and the mechanical difficulty of supplying dilute concentrations of gas to achieve a low analgesic state in the patient. Halogenated anesthetics such as trichloroethylene and methoxyflurane which are generally preferred cannot be used as analgesics with most present day equipment since the extremely dilute concentrations which are all that is necessary are not obtainable due to these anesthetics' great volatility in the liquid form. Delivery of analgesics or anesthetics by inhalation would however be prefered, if possible, as it allows the patients quick and total recovery without withdrawal symptoms commonly associated with narcotics delivered by injection. An increased number of patients have developed allergies to "local" analgesics and would therefore be benefited by the greater range available in inhalation analgesics. Such analgesics would further establish better patient rapport particularly among children by eliminating the fears associated with needle injections.
It will be apparent, therefore, that some of the difficult problems confronting medical personnel in administering inhalation narcotics have been those of accurately and reliably delivering specific concentrations of gas without waste or danger to operating personnel. While various mechanical expedients have been developed for reducing flow rates, safely venting excess anesthetic gas and regulating or monitoring equipment, none has been entirely satisfactory or adequate under all operating conditions. Hence, those concerned with the development and use of inhalation narcotic systems have recognized the need for relatively simple, safe, reliable and accurate system which will be economical in price as well as use.
A relatively uncomplicated method and apparatus for delivering gaseous anesthetics in a closed system of anesthesia has been previously disclosed in Moyat U.S. Pat. No. 3,183,906, issued May 18, 1965. By one embodiment of that method essentially pure anesthetic is introduced into a carrier gas from a "reversible physical absorbing agent" (activated carbon is given as a preferred type adsorbent) and delivered together with the carrier gas to the patient. In another aspect, the Moyat method involves a closed anesthesia system in which a patient's exhaled gas is recycled through the adsorbing agent containing the anesthetic, with the carbon dioxide exhaled by the patient being removed and fresh oxygen added to make up for that which was consumed.
Unfortunately, the Moyat method which appears to offer great advantages in economy, safety and simplicity compared to existing systems for delivering anesthetics, has not gained widespread acceptance. At least in part, this lack of acceptance may be due to the unsuitability of activated carbon for sustained and controllable release of adsorbed anesthetics. Both the capacity for adsorption of anesthetics and the delivery characteristics of activated carbon vary appreciably from batch to batch. Frequently the initial delivery rate of anesthetic from activated charcoal is greater under ambient temperatures than desirable for anesthesia, necessitating complicated control means to reduce concentration. the friable nature of charcoal, leading to the production of a fine charcoal dust, may also lead to problems of containment and possibly blockage of sensitive control devices.